✅ Simplify debugger probe test fixtures

[watson]

Motivation

Debugger domain specs were repeating full probe objects in many places, making tests noisy and harder to review. This also made it easy for test fixtures to drift from the probe shapes delivered by the UI/API.

Changes

• Added a shared createProbe() spec helper for browser-debugger tests.
• Replaced repeated probe literals in API, delivery API, and probe registry specs with the helper.
• Kept overrides only where the test intentionally varies that property.
• Made Probe.sampling optional to match the delivery schema/backend contract and existing runtime fallback handling.
• Updated assertions to use shared default function ids or probe-derived ids instead of duplicated literals.

Test instructions

• Run yarn test:unit --spec "packages/browser-debugger/src/domain/{api,probes,deliveryApi}.spec.ts"
• Verify the targeted debugger unit specs pass.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[watson]

• ✅ Simplify debugger API spec call sites #4741
• 🐛 Handle non-Error debugger throw values #4740
• ✅ Simplify debugger probe test fixtures #4736 👈 (View in Graphite)
• main

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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	171.86 KiB	171.86 KiB	0 B	0.00%	✅
Rum Profiler	7.88 KiB	7.88 KiB	0 B	0.00%	✅
Rum Recorder	21.21 KiB	21.21 KiB	0 B	0.00%	✅
Logs	54.31 KiB	54.31 KiB	0 B	0.00%	✅
Rum Slim	129.69 KiB	129.69 KiB	0 B	0.00%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 76.77% (+0.06%)

_{This comment will be updated automatically if new data arrives.
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